1. Field of the Invention
The present invention relates to the preparation of polymers based on boron and nitrogen, and to the use of such polymers in the production of ceramic materials and shaped articles based on boron nitride, especially boron nitride in fibrous form.
This invention also relates to certain novel compositions which are especially adopted for the synthesis of such polymers based on boron and nitrogen.
2. Description of the Prior Art
Boron nitride is a material increasingly in demand in this art, in light of its stability at elevated temperatures, its impact strength, its great chemical inertness and its very good thermal conductivity. Furthermore, its low electrical conductivity makes it an insulating material of choice.
Various processes are pressently known to the art for the preparation of boron nitride.
One such process includes reacting boron trichloride with ammonia in the gaseous state. A fine boron nitride powder is obtained in this manner, which may be sintered to produce solid shaped articles. However, the shaped articles thus produced exhibit a characteristic microporosity which may be highly detrimental for certain applications.
More recently, it was discovered that boron nitride could be produced by the pyrolysis of precursor polymers.
The advantage of this "polymer" method primarily resides in the form of the final product, and, more particularly, enables the production, after pyrolysis, of boron nitride fibers.
Thus, in U.S. Pat. No. 4,581,468 a preceramic organoboron polymer is described which is prepared by the interaction of ammoia (ammonolysis) with a B-trichloro-N-tris(trialkylsilyl)borazine (a cyclic compound) and which, as indicated, after drawing and pyrolysis at 970.degree. C., results in the production of boron nitride fibers.
However, the beginning cyclic compound described in this patent is difficult to prepare and is thus expensive. Consequently, it is not realistically suitable for production on an industrial scale.
More specifically, this cyclic compound is prepared by the thermolysis in xylene, at reflux, of a dichloro(trialkylsilylamino)borane having the formula: ##STR2##
Such a process presents the dual disadvantage, on the one hand, of giving a low yield (on the order of 20%), and, on the other, of requiring the use of an organic solvent with all of the necessary precautions and additional cost resulting therefrom (safety, elimination of the solvent, loss of solvent, etc.).
Finally, the maximum yield by weight of boron nitride that can be produced by pyrolysis of an organometallic polymer synthesized from said cyclic compound, may be insufficient.